Integrated circuits in general, and CMOS devices in particular, have continued to gain wide spread usage as demands for increased functionality and enhanced benefits continue to increase. In order to meet this demand, the integrated circuit industry continues to develop new circuit structures that enhance capabilities and extend the use of existing fabrication processes and equipment.
Key to the fabrication of integrated circuits is the ability to construct highly reliable and low resistance connections between the individual semiconductor device terminals themselves and other needed circuit components including various power supplying and common signal carrying conductor structures (buses). The industry often refers to connections made directly to a semiconductor device terminal as a contact (or windows) and connections made between other conductor structures as vias. The use of both tungsten and aluminum are well established in the art for use in this capacity.
The standard practice in the industry is to use tungsten plugs for contact (or window) and via-fill processes and Al-alloys (Al--Si, Al--Cu, Al--Cu--Si) for interconnect materials. Unfortunately, as contact and via dimensions decrease, their contact resistance to underlying material increases, especially with W-plugs (see Dixit et al., IEDM 1994). Consequently, two all-Al solutions have been advanced in the literature, (instead of w-plug/Al interconnect structure), that allow significant reductions in contact or via resistance over that obtained with W-plugs. The first solution is a force-fill method (Dixit et al., loc cit) and the second solution is a two-step Al reflow process (Zhao et al., IEDM 1996). However, process temperatures of approximately 450.degree. C. are required during the two-step reflow process for the aluminum to flow, form and adhere properly.
Current technology is expanding to the use of new dielectric materials that have a lower dielectric constant, such as polymer-based aerogels. Unfortunately, however, these polymer-based materials are easily damaged by the high processing temperatures (e.g., approximately 450.degree. C.) normally associated with the reflow processing of aluminum. Accordingly, these materials necessitate the use of lower process fabrication temperatures than are normally used with silicon-based dielectrics. Typically, process temperatures must not exceed a maximum of 350.degree. C. to avoid damaging these new dielectric materials. In an effort to lower the deposition temperatures typically associated with aluminum, some investigators have alloyed aluminum with germanium such as those disclosed in K. Kikuta and T. Kikkawa; J. Electrochem. Society; Vol. 143, No. 1, p. 228-232; January 1996. In such processes, the aluminum-germanium alloy is deposited in the contact opening or via. As is well known, pure aluminum melts at approximately 660.degree. C. Alloys of aluminum and germanium however, lower the melting point drastically. Unfortunately, germanium does not wet the dielectric material well and therefore requires a wetting layer of polysilicon to form and flow properly. Another problem with using a germanium-aluminum alloy is that the germanium precipitates epitaxially on silicon during subsequent processing since the crystal structures of the two are very similar. These problems have prevented wide spread commercial application of the aluminum-germanium alloy.
In summary, the use of aluminum to form contact and via interconnect structures has often resulted in unacceptably high contact resistance, alloy processing temperatures that are too high, particularly for polymer-based dielectrics, or other alloy characteristics that prevent successful commercial application.
Accordingly, what is needed in the art is an alloy forming interconnect process that operates successfully at temperatures low enough to prevent damage to desired polymer dielectrics.